Recently, there have been increasingly strong demands for electronic devices having ever smaller sizes and high reliabilities. In order to meet such demands, methods of mounting integrated circuits have been developed from methods of mounting IC packages on printed circuit boards as in the prior art to methods of patterning a conductor layer, resistors or the like mainly in a structure on a surface of an insulating board of ceramics for example, or directly mounting an IC chip on a board surface and hermetically sealing it.
In addition, according to demands for large scale integration or multiple function circuits, circuit boards have been made heretofore having circuits not only on the front surface of an insulating board but also on the rear surface thereof and including electrical connections of the circuits on the front and rear surfaces of the board, by means of through-holes formed in the board for passing electrical conductors therethrough. In such a circuit board having circuits on both the front and rear surfaces of an insulating board, which are electrically connected, it is particularly required of plug elements in through-holes formed in the board that:
a. the electrical connections between the circuits formed on the front the rear surfaces of the board, are achieved with sufficient certainty, and that
b. a sufficient airtightness be maintained so as not to lose the airtightness by means of the through-holes in the case where the circuit formed on either surface of the board must be hermetically sealed.
Conventionally, a method shown in FIG. 4 is used for forming conductive plug elements in through-holes formed in a board. The known method includes: forming prescribed through-holes 2 in an insulating board 1, squeezing metal paste 4 into the through-holes 2 using a rubber squeegee 3, and drying and sintering the metal paste in the through-holes 2, thereby forming metallic conductive plug elements in the through-holes 2. Then, as shown at A in FIG. 5, conductor patterns 5, 6 are formed on upper and lower surfaces of the conductive plug elements 4a filling the through-holes 2 of the insulating board 1, whereby the circuits on the front and rear surfaces of the insulating board 1 are electrically interconnected.
However, it is difficult to apply the above mentioned method where the through-holes are completely filled as shown at A in FIG. 5, since defects as described below often occur. For example, as shown at B in FIG. 5, a gap may be formed in a through-hole to cause a defective connection between the conductor pattern 5 on the upper surface of the board and the conductor pattern 6 on the lower surface thereof, or as shown at C in FIG. 5, the conductor pattern 5 on the upper surface and the conductor pattern 6 on the lower surface of the board are connected by a conductive portion 4a which does not fill the through-hole 2 resulting in a leaking connection. Thus, this method cannot completely satisfy the above mentioned requirements for the conductive plug element 4a in the through-holes 2, namely that a complete electric connection and a sufficient airtightness are assured.
There is another known method as shown in FIG. 6 and disclosed in Japanese Patent Publication No. 63-64918, wherein first prescribed through-holes 2 are formed in an insulating board 1. Next, a conductor pattern 6 is formed on the rear or lower surface of the board 1 to cover one opening of each through-hole 2. Then, a metallic wire 7 having a diameter slightly smaller than the inner diameter of each through-hole 2 and a length slightly shorter than the depth of each through-hole 2, is inserted into each through-hole 2 from the other open side. After that, a conductor pattern 5 is formed on the top of each metallic wire 7 to seal the other side of each through-hole 2.
However, this method also has a disadvantage in maintaining the desired airtightness of the through-holes 2. More specifically, the airtightness is maintained only by the conductor patterns 5, 6 formed on the upper and lower openings of the through-holes 2 and a gap unavoidably exists between the corresponding metallic wire 7 and the inner wall surface of the through-hole 2. Accordingly, if a small gap or crack occurs in the conductor patterns 5, 6 the airtightness is lost thereby. In addition, it is difficult to automate the work of inserting the metallic wires 7 into the through-holes 2, whereby the manufacturing efficiency is not good.
U.S Pat. No. 4,732,780 (Mitoff et al.) discloses a process for producing a hermetic feedthrough in a ceramic substrate by providing a sheet of liquid phase sinterable ceramic composition having a feedthrough hole, filling the feedthrough hole with refractory metal metallization material, firing the resulting structure to produce a sintered substrate and adherent metallization. The metallization includes continuous phases of refractory metal and glass. The refractory metal is contacted with electrically conductive intrusion metal and the resulting structure is heated to a temperature at which the glassy phase is fluid, the refractory metal is solid, and the intrusion metal is liquid whereby the liquid metal preferentially wets the refractory metal, migrates into the metallization displacing glass and, upon subsequent solidification, partially or wholly occupies the volume space originally containing the continuous glass phase.
U.S. Pat. No. 4,323,593 (Tsunashima) discloses a method of applying a paste through a hole in a printed circuit board. A mask is provided with a hole with an upper larger cross-section portion and a lower smaller cross-section portion, the junction between the portions forming a ring-shaped land. The lower hole portion has a cross-section larger than the cross-section of the hole in the printed circuit board. The printed circuit board is positioned adjacent the mask with the hole therein aligned with the hole in the mask, and paste is applied to the hole in the mask from the upper hole portion. This paste is pressed through the hole in the mask and fills the hole in the circuit board with paste and coats the surface of the circuit board which is toward the mask and/or the surface which is away from the mask in the area around and close to the hole with the paste.